The risks of a potential new diet pill and a 13-year-old weight-loss medicine face U.S. scrutiny next week as medical experts consider if the drugs' benefits outweigh possible side effects.

On Thursday, Arena Pharmaceuticals Inc goes before an advisory panel that will consider if the pill, lorcaserin or Lorqess, should win approval. The medicine is vital to Arena, a small company with no approved drugs on the market.

On Wednesday, the committee will decide whether to recommend tougher action against Abbott Laboratories Inc's diet drug Meridia on concerns about heart problems.

The advice on both pills will weigh heavily on the Food and Drug Administration, the agency that will make the final call.

Drugmakers have failed for decades to produce a pill to help people shed a significant number of pounds without serious side effects.

While earlier obesity pills failed to generate blockbuster sales, a new generation of drugs under FDA review this year could boost manufacturers looking to tap even a small slice of the nation's obese population.

When the FDA advisers meet on Arena, there's going to be a huge discussion on safety, predicted Hapoalim Securities analyst Jon LeCroy.

The FDA asked Arena to study if Lorqess might harm heart valves, a problem that led to the 1997 withdrawal of part of the fen-phen diet drug combination.

Lorqess, like the now withdrawn fenfluramine in fen-phen, is designed to block appetite signals in the brain. But the Arena drug is more selective in the receptors it affects, and the company says studies have not found heart problems linked to the medicine.

Debate at the panel may focus on whether the company's data are statistically strong enough to rule out a certain level of heart-valve damage, LeCroy said on Wednesday.

Arena faces a higher safety bar because the drug is not super effective, he added.

Like potential rivals, Lorqess is supposed to be used short term with diet and exercise. One year-long trial showed weight loss of 5.9 percent, compared with 2.8 percent with a placebo.

But patients regaining weight after they stop using the drug is a concern.

Being obese is a lifetime public health problem, and it isn't fixed by losing weight for six weeks, or a month or a year, consumer advocate Sidney Wolfe, head of Public Citizen's Health Research Group said recently.

The FDA's preliminary views on Lorqess are expected to become public on Tuesday when the agency is scheduled to release documents prepared for the panel meeting.

Two other companies, Vivus Inc and Orexigen Therapeutics Inc, are seeking to sell new diet drugs that combine medicines already approved for appetite suppression, seizures, addiction or other uses.

An FDA advisory panel in July shot down the Vivus drug, Qnexa, voicing concern about depression, memory loss, birth defects and other problems reported in studies. The company expects a final ruling by October 28.

That could clear the way for Arena or Orexigen to reach the market first. Arena has said an FDA decision on Lorqess is due by October 22. Orexigen does not face an FDA panel until December.

A new option could double the current diet drug market. Prescription and over-the-counter diet drugs altogether took in $381.5 million in 2009, according to data from IMS Health.

Arena has partnered with Eisai on Lorqess, while Orexigen has a $1 billion marketing deal with Takeda Pharmaceutical Co on Contrave, its weight loss drug.

Consensus forecast data from Thomson Reuters shows analysts expect Arena's drug to take in $62.8 million in 2011 sales and reach peak annual sales of $822 million.

Analyst LeCroy currently predicts approval of Lorqess in the second half of 2011 with sales of $600 million in 2015.

MORE WARNINGS FOR MERIDIA?

The older drug, Meridia, was approved in 1997.

Use dropped after data in November 2009 showed patients had more heart problems with Meridia than a placebo. In January, Abbott halted European sales of the pill after regulators said heart risks made the medicine too dangerous.

Although never a huge seller -- it took in $340 million worldwide in 2008 -- analysts expect the drug to see $101 million in global sales this year and $90 million in 2014, according to a consensus forecast by Thomson Reuters.

Consumer advocates and other critics have called for Meridia's withdrawal for years, saying the slight weight loss patients experience is not worth the risk.

Abbott says the drug is safe but should not be used by people with heart problems.

A study called Scout showed patients taking Meridia had a greater number of heart attacks and stroke than those given a placebo. Those taking the drug, also known as sibutramine, lost an average of 8.8 pounds, or less than 4 kilograms.

In January, the FDA asked Abbott to add a warning against Meridia's use by heart disease patients and said it would ask an advisory panel if more action was needed. The FDA has not said what steps it is considering, but options could include more warnings on the drug or its withdrawal from the market.

Nothing final has been determined, FDA spokeswoman Karen Riley said.

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